diff --git a/tests/50federation/30room-join.pl b/tests/50federation/30room-join.pl
index 24dc98ed3..b7df0a4fa 100644
--- a/tests/50federation/30room-join.pl
+++ b/tests/50federation/30room-join.pl
@@ -1055,10 +1055,7 @@ sub assert_is_valid_pdu {
          })->then( sub {
             my ( $event ) = @_;
 
-            # The joining HS (i.e. the SUT) should have invented the event ID
-            # for my membership event.
-
-            # TODO - sanity check the $event
+            assert_json_keys( $event->{content}, qw( membership ) );
 
             Future->done(1);
          }),